“if you see the Buddha upon the road, kill him.”

What it means is that, during meditation, if images and thoughts of Buddha show up in our minds while we are meditating, we should not fixate on these thoughts and images (kind of hard to do when there is at least one, if not several statues or pictures of Buddha in the room).

The point of Zen meditation is to breathe, focus on the breath, and not attach to any thought or image at all, just let them pass through the mind.

This is very important. That’s why the saying about killing the Buddha is so extreme. It’s to remind us to avoid mental attachment, especially while meditating.